1. Field of the Invention
The present invention pertains to an improved method for welding high chromium duplex stainless steel with relatively large diameter filler metal rod while retaining material toughness and corrosion resistance in the weld and the heat affected zone and adhering to established engineering standards.
2. Background
Duplex stainless steels are alloys of iron, chromium and nickel with a micro-structure consisting substantially of ferrite and austenite or ferrite and martensite. Unique features that distinguish duplex stainless steels from other classes of stainless steels are their ability to accommodate a high chromium and molybdenum content. These elements control pitting and crevice corrosion resistance, have good resistance to chloride stress corrosion and high strength. In this regard, chrome duplex stainless steels are ideal for certain oil and gas field applications such as hot seawater processing systems and similar systems which handle generally corrosive fluids under relatively high pressures. Accordingly, the use of these materials is advantageous in complex fluid distribution systems such as are found in oil field production fluid gathering systems and enhanced recovery operations such as water flood systems. These systems include complex piping networks and manifolds which require substantial fabrication, most of which is done by manual as well as machine welding.
One consideration in using duplex stainless steel for corrosive fluid handling piping and vessels is the time required to fabricate the systems, in particular the time required to perform welds between various pipe sections, for example. Since it is generally accepted that the micro-structures of duplex stainless steels are, in general, notably changeable with heating such as by welding, and that these changes in micro-structure influence the corrosion resistance of the material, welding methods have been required to be modified to prevent excessive heating of the weld area. This, generally, has required that relatively small diameter weld filler wire or rod be used in the welding operation to minimize heat generated at the weld. Accordingly, the weld material deposited in the weld area per pass of the filler material is relatively low and several passes, with interpass cooling of the weld, have been required to prevent over-heating and consequent decrease of the corrosion resistance and toughness of the material.
However, with this background in mind, an improved method of welding duplex stainless steels has been developed wherein rather unexpected improvements in welding speed and reduced fabrication time have been realized while adhering to certain specifications of desired material properties such as impact toughness and corrosion resistance. It is to this end that the present invention has been developed with a view to providing substantial improvements in time required for fabricating piping and similar structural members by welded chrome duplex stainless steels, in particular.